The present invention relates to a power-assisted steering system in a wheeled vehicle capable of effecting the driver's steering effort in accordance with the driving condition of the vehicle, and more particularly to an improvement of a power-assisted steering system of the type which comprises a servo valve associated with a hydraulic power cylinder to interconnect a hydraulic pressure source and the power cylinder and being responsive to rotary motion of a steering shaft to selectively direct the flow of fluid under pressure from the pressure source to one of opposite fluid chambers of the power cylinder and to permit the flow of fluid from the other fluid chamber to a fluid reservoir, thereby to effect reciprocating movement of a power piston in the cylinder, and an electrically operated bypass valve disposed within a bypass passage to permit bypass flow of fluid passing therethrough between the pressure source and the fluid reservoir in its energized condition.
In such a conventional power-assisted steering system as described above, the bypass valve is deenergized to effect a sufficient hydraulic power assist during low speed travel of the vehicle so as to lighten the driver's steering effort and is energized to decrease the hydraulic power assist during medium of high speed travel of the vehicle so as to increase the driver's steering effort, for stability in turning maneuvers. For such control of the bypass valve, there have been proposed power-assisted steering systems of the vehicle speed responsive type and of the engine rotation responsive type. In these power assisted steering systems, however, it is desireable to control the hydraulic power assist in response to deceleration and shifting operation of the power transmission during cornering travel of the vehicle. This results in unstable turning maneuver during cornering travel of the vehicle at a high speed or on a descending road.
In FIG. 9, there is illustrated such a cornering travel path of the vehicle for passing through a curve D-E on a travel road. Assuming that a vehicle equipped with a manual transmission is operated to pass through the curve D-E, a skilled driver will conduct operation of the vehicle as shown in FIG. 10 to pass through the curve D-E at a high speed, while an unskilled driver will conduct operations of the vehicle as shown in FIG. 11 to pass through the curve D-E at a low speed. In the operation of FIG. 10, the brake pedal is depressed at a point A of the road to decelerate the vehicle, the clutch pedal is depressed at a point B to conduct shift-down operation from 4th speed to the 3rd speed, the accelerator pedal is depressed at a region between C-F to conduct cornering travel of the vehicle while in 3rd speed, and subsequently the clutch pedal is depressed at the point F to conduct shift-up operation from the 3rd speed to the 4th speed so as to conduct straight travel of the vehicle in 4th speed from a point G. During such operation, variation of the rotation speed (n) of the prime mover, the throttle opening degree (.theta.), the amount of intake air (.theta./n) and the vehicle speed (V) will occur as shown in the figure. In the operation of FIG. 11, the brake pedal is depressed at the point E to decelerate the vehicle, and simultaneously the clutch pedal is depressed to conduct shift-down operation from the 4th speed to the 3rd speed. During such cornering travel of the vehicle, variation of the rotation speed (n) of the prime mover, the throttle opening degree (.theta.), the amount of intake air (.theta./n) and the vehicle speed (V) will occur in the figure.
As is understood from the above description, it is desirable that in the operation of FIG. 10, the driver's steering effort is controlled as shown by a solid line in FIG. 12, and it is also desirable that in the operation of FIG. 11, the driver's steering effort is controlled as shown by a dot and dash line in FIG. 12. However, in use of the conventional power-assisted steering system of the vehicle speed responsive type, the driver's steering effort in the operation of FIG. 10 will be controlled as shown by a solid line in FIG. 14. This means that when the vehicle is operated by the skilled driver, the driver's steering effort is excessively decreased at the region of points C-F to cause an unstable turning maneuver. In use of the conventional power-assisted steering system of the engine rotation responsive type, the driver's steering effort in the operation of FIG. 10 will be controlled as shown by a solid line in FIG. 15 or FIG. 16. This means that the driver's steering effort is excessively decreased at the regions of A-C and F-G or the regions of B-C and F-G to cause unstable turning maneuver.